Convert radios on set-sender page

Includes making the legend the page header to
follow design system guidance:

https://design-system.service.gov.uk/get-started/labels-legends-headings/

I had to add some override styles to allow the
legend to classes as a full-width column, and the
radios to be in one that is three-quarters.

app/assets/stylesheets/govuk-frontend/overrides.scss

@@ -46,3 +46,9 @@
 .govuk-button {
   margin-bottom: 0px;
 }
+
+// Allow legends that contain the h1 to go full width
+.govuk-fieldset__legend.govuk-grid-column-full {
+  // $class parameter is deprecated but needed for v2 of GOVUK Frontend. Remove for v3 & above
+  @include govuk-grid-column(full, $class: false);
+}